Arguments

a logical (not NA) which indicates whether to
capture the output of the command as an R character vector.

ignore.stdout, ignore.stderr

a logical (not NA)
indicating whether messages written to ‘stdout’ or
‘stderr’ should be ignored.

wait

a logical (not NA) indicating whether the R
interpreter should wait for the command to finish, or run it
asynchronously. This will be ignored (and the interpreter will
always wait) if intern = TRUE. When running the command
asynchronously, no output will be displayed on the Rgui
console in Windows (it will be dropped, instead).

input

if a character vector is supplied, this is copied one
string per line to a temporary file, and the standard input of
command is redirected to the file.

timeout

timeout in seconds, ignored if 0. This is a limit for the
elapsed time running command in a separate process. Fractions
of seconds are ignored.

show.output.on.console, minimized, invisible

arguments
that are accepted on Windows but ignored on this platform, with a
warning.

Details

This interface has become rather complicated over the years: see
system2 for a more portable and flexible interface
which is recommended for new code.

command is parsed as a command plus arguments separated by
spaces. So if the path to the command (or a single argument such as a
file path) contains spaces, it must be quoted e.g. by
shQuote.
Unix-alikes pass the command line to a shell (normally ‘/bin/sh’,
and POSIX requires that shell), so command can be anything the
shell regards as executable, including shell scripts, and it can
contain multiple commands separated by ;.

On Windows, system does not use a shell and there is a separate
function shell which passes command lines to a shell.

If intern is TRUE then popen is used to invoke the
command and the output collected, line by line, into an Rcharacter vector. If intern is FALSE then
the C function system is used to invoke the command.

wait is implemented by appending & to the command: this
is in principle shell-dependent, but required by POSIX and so widely
supported.

When timeout is non-zero, the command is terminated after the given
number of seconds. The termination works for typical commands, but is not
guaranteed: it is possible to write a program that would keep running
after the time is out. Timeouts can only be set with wait = TRUE.

Timeouts cannot be used with interactive commands: the command is run with
standard input redirected from /dev/null and it must not modify
terminal settings. As long as tty tostop option is disabled, which
it usually is by default, the executed command may write to standard
output and standard error. One cannot rely on that the execution time of
the child processes will be included into user.child and
sys.child element of proc_time returned by proc.time.
For the time to be included, all child processes have to be waited for by
their parents, which has to be implemented in the parent applications.

The ordering of arguments after the first two has changed from time to
time: it is recommended to name all arguments after the first.

There are many pitfalls in using system to ascertain if a
command can be run — Sys.which is more suitable.

Value

If intern = TRUE, a character vector giving the output of the
command, one line per character string. (Output lines of more than
8095 bytes will be split.) If the command could not be run an R
error is generated.
If command runs but gives a non-zero exit status this will be
reported with a warning and in the attribute "status" of the
result: an attribute "errmsg" may also be available.

If intern = FALSE, the return value is an error code (0
for success), given the invisible attribute (so needs to be printed
explicitly). If the command could not be run for any reason, the
value is 127 and a warning is issued (as from R 3.5.0).
Otherwise if wait = TRUE the value is the exit status returned
by the command, and if wait = FALSE it is 0 (the
conventional success value).

If the command times out, a warning is reported and the exit status is
124.

Stdout and stderr

For command-line R, error messages written to ‘stderr’ will be
sent to the terminal unless ignore.stderr = TRUE. They can be
captured (in the most likely shells) by

system("some command 2>&1", intern = TRUE)

For GUIs, what happens to output sent to ‘stdout’ or
‘stderr’ if intern = FALSE is interface-specific, and it
is unsafe to assume that such messages will appear on a GUI console
(they do on the macOS GUI's console, but not on some others).

Differences between Unix and Windows

How processes are launched differs fundamentally between Windows and
Unix-alike operating systems, as do the higher-level OS functions on
which this R function is built. So it should not be surprising that
there are many differences between OSes in how system behaves.
For the benefit of programmers, the more important ones are summarized
in this section.

The most important difference is that on a Unix-alike
system launches a shell which then runs command. On
Windows the command is run directly – use shell for an
interface which runs commandvia a shell (by default
the Windows shell cmd.exe, which has many differences from
a POSIX shell).

This means that it cannot be assumed that redirection or piping will
work in system (redirection sometimes does, but we have seen
cases where it stopped working after a Windows security patch), and
system2 (or shell) must be used on Windows.

What happens to stdout and stderr when not
captured depends on how R is running: Windows batch commands behave
like a Unix-alike, but from the Windows GUI they are
generally lost. system(intern = TRUE) captures ‘stderr’
when run from the Windows GUI console unless ignore.stderr =
TRUE.

The behaviour on error is different in subtle ways (and has
differed between R versions).

The quoting conventions for command differ, but
shQuote is a portable interface.

Arguments show.output.on.console, minimized,
invisible only do something on Windows (and are most relevant
to Rgui there).

See Also

Examples

# list all files in the current directory using the -F flag
## Not run: system("ls -F")
# t1 is a character vector, each element giving a line of output from who
# (if the platform has who)
t1 <- try(system("who", intern = TRUE))
try(system("ls fizzlipuzzli", intern = TRUE, ignore.stderr = TRUE))
# zero-length result since file does not exist, and will give warning.